fix(metro-config): Remove extraneous Babel pass

[kitten]

Closes ENG-10870

Why

We want to remove an extraneous Babel pass in the metro-transform-worker, as a follow-up task to #25833. The relevant todo is in the worker itself:

expo/packages/@expo/metro-config/src/transform-worker/metro-transform-worker.ts

Lines 222 to 223 in b80decc

	// TODO: This MUST be run even though no plugins are added, otherwise the babel runtime generators are broken.
	// if (plugins.length) {

Previously, we couldn't remove this pass as without it dev: false builds would be missing _interopRequireWildcard and _interopRequireDefault Babel helper inline functions.

However, removing it gets rid of an unnecessary transformFromAstSync call which may run without plugins and clones the entire Babel Input AST.

How

Investigating the issue, it turns out that Babel has a cache for AST nodes that keeps track of previously created path and scope objects. This is kept on @babel/traverse's traverse.cache.

The cloneInputAst: true option was needed on at least one transformFromAstSync because it busts this path/scope cache. It internally performs a simple deep-clone of the AST and means that the next traverse call doesn't have a path/scope cache to fall back on. This will cause Babel to recrawl the scope of an AST.

This basically means that this problem has two sides to it, which work together to cause the bug that removing cloneInputAst: true triggers:

1. A Babel plugin in our babel-transformer is corrupting the scope early on (as part of @react-native/babel-preset)
2. A Babel plugin in one of our transformFromAstSync passes is corrupting the AST due to the corrupted scope.

In our case, the Babel plugin corrupting the scope is @babel/plugin-transform-modules-commonjs. This plugin uses hub.addHelper("interopRequireWildcard") and similar calls that inject Babel helpers. In most cases, these calls to hub.addHelper don't update any referencePaths and thus the function declarations it injects are un-referenced from the perspective of Babel’s scope.

The Babel plugin corrupting the AST and removing the above function declarations is Metro’s constantFoldingPlugin, which is only used when dev: true is set. This plugin, among other things, removes unused declarations. When it sees the inserted function declarations with the corrupted scope, it removes them.

There are several ways to ensure that the scope is correct after the initial Babel plugins are run, either:

• each Babel plugin that uses hub.addHelper (or inserts declaration without letting traversal continue) must update the scope manually.
  • This bug may go away in Babel 8 and we don't control all plugins that a user may use, so this isn't a suitable fix
• the AST may be cloned using cloneInputAst: true on the constantFoldingPlugin instead, or the via a deepClone utility.
  • Basically, we could also move cloneInputAst: true to the dev: true pass.
• traverse.cache.clear() may be called to clear the entire traversal cache
  • This could have performance implications that are hard to analyse.
• programPath.scope.crawl() can be called in or before the constantFoldingPlugin to make sure it always has an up-to-date scope.

This PR, when applied, uses the last solution in the above list.
It also disables cloneInputAst: true on all transform passes, since only the constantFoldingPlugin uses the scope to alter the AST in a way that could break the output code.

Test Plan

• A new unit test has been added for metro-transform-worker that uses the constantFoldingPlugin and checks the output for code that will generate a _interopRequireWildcard declaration
  • This ensures we don't break the constantFoldingPlugin again without having to run E2E tests
  • I also used this unit test to test that reverting this fix breaks, as expected
• I manually tracked down the bug by disabling cloneInputAst everywhere and then selectively stepping through code. Removing constantFoldingPlugin also fixes the output.
• By tracking down some related Metro issues, I tested traverse.cache.clear() in conjunction with other changes

Checklist

• Documentation is up to date to reflect these changes (eg: https://docs.expo.dev and README.md).
• Conforms with the Documentation Writing Style Guide
• This diff will work correctly for npx expo prebuild & EAS Build (eg: updated a module plugin).

[linear]

ENG-10870 Remove extraneous babel pass in metro transformer

[EvanBacon]

Finally had time to dig through this extensively today, genius work here.

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